This invention pertains to power measuring apparatus and more particularly to such apparatus for measuring power drawn for local utilities.
One of the problems of AC power measurement in existing power installations, requiring the measurement of AC current, is that of overcoming the physical size limitations of a pick-up transducer. Very little space is available in switching/breaker boxes or terminals for the addition of current transformers. Further, the problem of using non-technical personnel to install such devices requires simplicity of the task and built-in safety features regarding installation. Finally, the transducer must be inexpensive, reliable and reasonably precise from a physical measurement point of view to be useful.
In order to attain measurement errors not exceeding .+-.0.25% full-scale using ferromagnetic core materials in a current transformer, core dimensions would approximately be 11".times.8".times.3" (outside) and would weigh in the order of 23 pounds, using the most efficient configuration . . . for 1000 amp RMS applications. Further, in order to meet such stringent accuracy requirements, the air-gap must be precisely held, and the core must be carefully oriented perpendicular to the primary feed-through turn, which in turn must be placed at the geometrical center of the core.
On the other hand, the use of nonferrous cores has not been successful in the past because transformation efficiencies were considered to be too low at power frequencies.
Even when the transformer problem is solved current-voltage multipliers must be used. Heretofore people have proposed a combination logarithmic amplifiers and summing amplifiers or analog to digital converters with microprocessors. Either concept while workable is relatively expensive.
The idea to use Hall-effect devices has been proposed because they are efficient as multiplication devices. However, the application of a current sensors is impractical since flux concentrators must be employed.
Many years ago there were proposed square-law multipliers. Typical of these proposals are in U.S. Pat. No. 1,586,533 issued June 1, 1926 and "An Electron Tube Wattmeter and Voltmeter and Phase Shifting Bridge" Proc. IRE; vol. 30, October 1930 at page 1743. These proposals suffered from severe limitations and could not be realized with any amount of accuracy, reproducibility or economy.